The present invention relates to intumescent coatings, such as those used as paints and the like, and to methods utilizing such coatings for fire retardance. For structures fabricated from polymeric materials and the like, flame spread and thermal decomposition generally is accelerated to the point that normal fire fighting response times are inadequate to prevent significant combustion of the structure. Accordingly, intumescent coatings have been employed as paints to form a protective layer on the surface of such structures. As the name implies, intumescent coatings bubble when exposed to flames and produce an insulative layer of char and char foam.
The relatively low tendency for elemental carbon to oxidize has led to its incorporation into intumescent coatings. In this regard, highly-flammable substrates can be thermally-protected by application of a surface coating having ingredients catalyzed to be pyrolized into a carbonific char. Thus, the coating is converted from an oxidizable composition into elemental carbon. Broadly, such coatings comprise a polyhydric organic compound, an acid forming catalyst, and a blowing agent which intumesces the carbonific char formed from the acid-catalyzed pyrolysis of the polyhydric compound into a carbonific char foam having a relatively low thermal conductivity.
On its own, a pure carbon foam would not be expected to provide optimal thermal insulation. Consequently, the carbon foam often is supplemented with inorganic additives. Some inorganics, e.g., titanium dioxide and zinc oxide, provide nucleating sites for gas formation, resulting in fine cell, low density foam. Inasmuch as heat conduction through a gas can be several orders of magnitude lower than through a solid, a low density foam provides insulation improved over a higher density foam, especially when coated on a heat-conducting substrate such as a metal or a carbon-fiber composite. Additionally, several inorganics, including zirconium salts, borates, phosphates, and titanium dioxide, can contribute to the formation of a refractory layer over the top of the foam layer. The refractory layer advantageously provides an inert, highly infrared reflective layer, which can contribute significantly to the insulative properties of the foam. The infrared reflectance of the char also may be improved by the addition of inorganics having low emissivities such as titanium dioxide, zirconium dioxide, phosphate and antimonate glass. Other inorganics such as silica microballoons and silicone resins may be added to decrease heat conduction through the foam.
To form intumescent coatings having good integrity and offering resistance to high humidity, water-insoluble intumescent or char-forming agents, including selective salts of nitro aromatic amine compounds such as 4,4'-dinitrosulfanilimide, have been blended with epoxy-polysulfide or epoxy-chlorosulfonated polyethylene binder systems. Although such formulations are efficient intumescents, their efficiency is not optimized because the nitro aromatic intumescent species produces an exothermic char-forming reaction. To counter this exothermic effect, ablatives and endothermic fillers, including zinc borate and hydrated endothermic fillers such as aluminum hydroxide pigments, are beneficial.
The intumescent coatings heretofore known in the art have been used successfully to effectively reduce flame spread and to protect substrates from thermally-induced mechanical/chemical decomposition. Although these coatings have demonstrated significant fire retardant properties, they generally must be used at thicknesses, e.g., .gtoreq.5.08 mm (.gtoreq.200 mils), which precludes their use in applications where weight requirements are a consideration. Moreover, conventional coatings are not formulated to withstand exposure to the severe environments common in marine applications and the like, and often have ingredients which leach out after extended immersion in seawater. For certain applications, the intumescent reaction temperature of conventional formulations may be at or near the service temperature. Accordingly, there has existed and remains a substantial need for intumescent coating systems which not only will afford the requisite thermal insulation at minimum coating thicknesses and at elevated service temperatures, but which also will retain an insulative capability even after exposure to harsh marine environments and the like.